During activation of an anti-lock braking system (ABS), the brake motor controller may request or command torque that exceeds the motor's available output torque, leading to slow ABS response time and poor ABS performance. This result is especially likely under conditions of high motor output shaft speed and low motor supply voltage. In ABS using permanent magnet brushless, switch reluctance, or any other type of an electric motor to apply brake caliper pressure, the motor's output torque capacity can be increased through phase advancing the commutation angle, i.e., so-called "field weakening." Such techniques, however, may cause rough transitions and instability when the vehicle is operating at the interface between normal braking and ABS activation. The present invention avoids these problems by including a predictor in the control scheme of the brake-by-wire system to anticipate the need for phase advance commutation in determining the value of the torque requested or commanded from the motor. The invention is based upon two principles:
The field strength, and therefore the motor torque constant, can be electronically controlled in electric motors by modifying conduction angles. PA1 The conduction angles required for a particular torque and speed condition can be estimated using predictive signal processing techniques.
The first bullet point above can be better understood by examining FIG. 1, which shows the torque-speed trajectory of a typical electric motor during an ABS transient. When ABS is activated at time =0.0 seconds, the torque vs. speed characteristic for the electric motor is zero speed and zero torque as the motor begins to accelerate to reach a commanded torque value corresponding to a brake caliper apply pressure of typically 20 kN. It is desirable that the motor reach this output torque, and actuate the caliper apply this pressure, in a minimum amount of time. Without modifying the conduction angles (i.e., constant field control), the motor and thus the caliper pressure is constrained to the f1-torque curve. A typical brake-by-wire system with constant field control would require 0.220 sec. to reach the typical commanded caliper pressure of 20 kN. Modification of the conduction angles provides curves f2 through f5. As seen in FIG. 1, modification of the conduction angles can reduce the response time from the 0.220 sec. response time of f1 to the approximately 0.14 sec provided by f5.
Although the torque-speed characteristic of an electric motor can be modified continuously through field control, i.e., through continuous modification of conduction angles, such control proves disadvantageous because the motor current is increased significantly in the case of curve f5 compared to the case of curve f1. Higher motor current results in lower motor operating efficiency and greater thermal heating of the brake caliper. Thus, it is desirable to modify the conduction angles by the minimum amount required to provide adequate time response, and thus maximize the motor's efficiency. For the simple case of a constant torque command shown in FIG. 1, the minimum or optimum conduction angle modification is easily determined. In general, however, determination of the optimum conduction angle modification is complicated by the fact that the motor torque command is not constant. On the contrary, it varies continuously in a random-like fashion. The motor controller requests or communicates the motor torque command through a CAN serial link at 10 msec intervals.
Fortunately, as suggested by the second bullet point above, optimum conduction angles can be determined by using predictive signal processing techniques to estimate the motor torque value that will be commanded at the next 10 msec CAN interval. The maximum instantaneous motor torque available at the then current conduction angle is computed then compared to the predicted motor torque command. This error is used to adjust the conduction angles in a manner that meets the anticipated torque requirements without unnecessarily increasing average motor current.